Interpretive Summary: The human pathogen Campylobacter jejuni lives in the gastrointestinal tract (gut) of warm blooded animals. In the gut, the availability of oxygen is limited; therefore, less efficient electron acceptors such as nitrate or fumarate are used by C. jejuni. C. jejuni has a highly branched respiratory chain and gene expression profiling indicates that a large number of genes are differentially expressed in vivo compared to in vitro growth conditions. The regulation mechanisms behind these alterations are however a mystery as no homologues exists in C. jejuni of transcription factors known to regulate the energy metabolism in other bacteria. Here we demonstrate by transcription analysis and enzyme assays that the response regulator RacR is regulating a number of genes involved in the synthesis and use of fumarate as electron acceptor. The activity of RacR is regulated by the cognate sensor RacS as demonstrated by phosphorylation studies and is dependent on the available electron acceptor. Footprinting assays showed that RacR binds directly to 5 promoter elements. Our results indicate that the failure of the C. jejuni racR::cm mutant to colonize chickens may be due to the inability of optimal nutrient utilization in the presence of alternative electron acceptors.

Technical Abstract:
The natural environment of the human pathogen Campylobacter jejuni is the gastrointestinal tract of warm blooded animals. In the gut, the availability of oxygen is limited; therefore, less efficient electron acceptors such as nitrate or fumarate are used by C. jejuni. C. jejuni has a highly branched respiratory chain and gene expression profiling indicates that a large number of genes are differentially expressed in vivo compared to in vitro growth conditions. The regulation mechanisms behind these alterations are however a mystery as no homologues exists in C. jejuni of transcription factors known to regulate the energy metabolism in other bacteria. Here we demonstrate by transcription analysis and enzyme assays that the response regulator RacR is regulating a number of genes involved in the synthesis and use of fumarate as electron acceptor. The activity of RacR is regulated by the cognate sensor RacS as demonstrated by phosphorylation studies and is dependent on the available electron acceptor. Footprinting assays showed that RacR binds directly to 5 promoter elements. Our results indicate that the failure of the C. jejuni racR::cm mutant to colonize chickens may be due to the inability of optimal nutrient utilization in the presence of alternative electron acceptors.